1. Field of the Invention
The present invention relates to the fabrication of semiconductor materials for application in optoelectronic devices operating in the infrared portion of the electromagnetic spectrum. Specifically, the repeated and deliberate deposition of thin layers such as indium arsenide (InAs), indium antimonide (InSb), gallium arsenide (GaAs) or gallium antimonide (GaSb), not necessarily in this order, forms a superlattice. This superlattice exhibits specific material properties, such as effective semiconductor bandgap energy, and these properties may be tailored by changing the individual layer thicknesses and/or layer constituents comprising said superlattice. The present invention may be employed in electronic and photonic devices used to detect and convey the presence of, or create, energy in the form of infrared light.
2. Prior Art                Scientific analysis and discussion of type-II superlattices relevant to the present invention are disclosed in the following publications, which will be referred to below:        1. D. L. Smith and C. Mailhiot, “Proposal for Strained Type II Superlattice Infrared Detectors,” Journal of Applied Physics 62, p. 2545 (1987).        2. “Future Trends in Microelectronics: The Nano Millennium,” edited by Luryi, Xu, and Zaslaysky, ISBN-10 #0471212474 (2002).        3. J. Kubacka-Traczyk, I. Sankowska, J. Kaniewski, “Interface Influence on Structural Properties of InAs/GaSb type-II Superlattices,” Optica Applicata, Vol. XXXIX, No. 4, p. 875 (2009).        4. H. J. Haugan, L. Grazulis, G. J. Brown, K. Mahalingam, D. H. Tomich, “Exploring Optimum Growth for High Quality InAs/GaSb Type-II Superlattices,” Journal of Crystal Growth 261(4), pp. 471-478 (2004).        5. J. B. Rodriguez, P. Christol, L. Cerutti, F. Chevrier, A. Joullié, “MBE Growth and Characterization of Type-II InAs/GaSb Superlattice for Mid-infrared Detection,” Journal of Crystal Growth 274(1-2), pp. 6-13 (2005).        6. Y. Chen, A. Moy, K. Mi, and P. Chow, “High Performance Type II Superlattice Photo Diodes for Long Wavelength Infrared Applications,” Proc. of the 16th International Conference on Molecular Beam Epitaxy, Berlin, Germany, Aug. 22-27, 2010.        
The present invention herein improves on the layered structure known as an indium arsenide (InAs) and gallium antimonide (GaSb) type-II superlattice. The basic concept of the superlattice itself, as applied to semiconductors, was described by Esaki, Ludeke and Tsu in U.S. Pat. No. 3,626,257 (1971). The foundation of their approach was to join a plurality of successive material layers so as to create a structure that exhibited a one dimensional spatial periodic variation in its band-edge energy. As science and engineering evolved to enable the realization of novel assemblies, Mailhiot and Smith disclosed a superlattice comprised of InAs and GaSb in an academic publication [reference 1 above]. Mailhiot and Smith described creating an artificial material comprising adjacent, alternating layers of InAs and GaSb. The particular constituents of this layered structure, those being InAs and GaSb, create a semiconductor material where the electronic energy band structures in adjacent layers form what is classified as a type-II interface, and such a structure is referred to as a type-II superlattice. Moreover, Mailhiot and Smith described that changing the thicknesses of the comprising layers within this type-II superlattice affects the effective bandgap energy exhibited by the material. Further discussion on the theory of this material is presented in the chapter “Infrared Detectors Based on InAs/GaSb Superlattices” from the book of reference 2 above. Such InAs/GaSb type-II superlattice structures have particular application to devices which interact with low energy photons classified in the midwave-infrared (MWIR), longwave-infrared (LWIR) and very longwave-infrared (VLWIR) portion of the electromagnetic spectrum.
Other inventions have been previously disclosed which combine multiple semiconductor layers for application to devices interacting with infrared photons, but these inventions rely on different scientific principles, or exhibit deleterious qualities, than the present invention. Raymond Chin in U.S. Pat. No. 4,450,463 (1984) described the assembly of layers to form multiple quantum wells. Because of the distinct energy levels which exist in such quantum wells, only photons within certain energy ranges could be absorbed by the quantum well, resulting in the promotion of electrical charge carriers which are then utilized to convey the detection of said photons. Quantum well infrared photodetectors (QWIP), such as the invention of Chin, only absorb photons arriving from a non-normally incident direction within the quantum well regions, and tend to be better suited for systems operating in the MWIR.
Razeghi in U.S. Pat. No. 6,864,552 described a focal plane array device employing the type-II superlattice structure previously presented by Malhiot and Smith. In Razeghi the specific materials disclosed in comprising the superlattice layers were InAs/GaSb, SiGe, InAs/Ga(x) In(1−x)Sb, and InAs/GaSb/AlSb. This limitation of specific layers with which to form the type-II superlattice creates a narrowed constraint in the design parameters of the realized device.
The presence of InAs and GaSb layers periodically adjacent to one another is only a subset of the qualities which define and affect the operating characteristics of a device employing such a type-II superlattice. The boundary between InAs and GaSb layers, that is the atomic bonding of specific atoms, occurs in one of two primary forms, these forms being a GaAs-like interface or a InSb-like interface. It is well known that the InSb-like interface plays an important role in achieving high quality InAs/GaSb superlattice materials grown on GaSb substrate, such as improving InAs/GaSb interface quality and balancing tensile strain introduced by InAs layer [reference 3 above]. Assertions have been presented which conclude that the type-II superlattice quality was negatively affected when the InSb layer was greater than 1 atomic monolayer [references 4 and 5 above].